It is known that the optical characteristics of liquid crystal cells are temperature-dependent. For example, the threshold potential of twisted nematic liquid crystal displays (usually abbreviated to TN-LCD) decreases with increasing temperature. This dependence is disadvantageous above all for multiplex applications.
On the other hand, liquid crystal materials for TN-LCDs are frequently treated with an optically active component in order to avoid the reversal of the twisting direction ("reverse twist"). The threshold potential V.sup.P.sub.th of TN-LCDs when such a material doped with an optically active component is used may be approximated from the following equation: EQU V.sup.p.sub.th .apprxeq.V.sup.o.sub.th (1+F.multidot.d/.vertline.p.vertline.).sup.1/2 ( 1)
in which V.sup.o.sub.th denotes the threshold potential when the undoped material is used, d is the plate separation (that is, the layer thickness of the liquid crystal material), .vertline.p.vertline. denotes the absolute value of the natural pitch of the doped liquid crystal material and F is a function of the elastic contants and the twisting in the cell [Appl. Phys. Lett. 25, 12 (1974) and IEEE Trans. Electron Devices ED-10, 141 (1974)].
From equation (1) it follows that the decrease in the threshold potential can be compensated for wholly or partially by using optically active doping materials which induce a decreasing absolute value of the pitch p with increasing temperature, similar considerations also apply to liquid crystal cells such as STN-LCDs ("supertwisted nematic"), SBE-LCDs ("supertwisted birefringence effect"), OMI-LCDs ("optical mode interference") and the like.
From DE-A-2827471, Z. Naturforschung 34a, 594 (1979) and Phys. Lett. 78A., 285 (1980) it is known that a decreasing value of .vertline.p.vertline. with increasing temperature can be achieved by adding at least two suitable, optically active doping materials in a fixed ratio insofar as one of the doping materials in the nematic carrier substance produces a right-handed twisting and the other produces a left-handed twisting. However, such a doping has the disadvantage that two or more optically active doping materials are required and in general even small deviations in the ratio lead to large variations in the temperature pattern of the threshold potential and the pitch. Moreover, relatively high total concentrations of doping materials are usually required.
However, only few individual compounds having the desired property are known and the decrease of .vertline.p.vertline. with increasing temperature is limited as a rule to a narrow or unfavourable temperature range, whereby the display quality or the utilizable temperature range is distinctly reduced. EP-A-0211646 discloses, for example, compounds having two similar centers of chirality, which have the desired temperature dependence above 0.degree. C., but which lead to helix inversion between about -20.degree. C. and 0.degree. C.